The invention relates to grids for holding fuel rods in a nuclear fuel assembly, and in particular to grids used in pressurized water reactors and serving to hold rods at the nodes of a regular array, generally a square array.
The invention relates in particular to making straps or plates which are assembled together to make up such grids, generally via half-depth slots.
The grids of an assembly define common cells which receive the rods and other cells having guide tubes passing therethrough, which tubes are often welded to the grids. Conventionally, in grids used for holding rods longitudinally, rigid dimples or bosses for abutment with the rods are provided in two of the four faces of a common cell for receiving a rod, and two holding springs are cut out from or fitted to the other two faces so as to press a rod onto the dimples. Between grids designed to hold fuel rods at the nodes of a regular array, it is possible to interpose grids provided with fins that serve only to improve mixing of coolant streams flowing along the assembly.
It is important to reduce the headloss caused by the presence of the grids. For this purpose, proposals have already been made to round those edges of the straps which are situated upstream in the flow direction (EP-A-0 273 183). However, that convex shape gives rise to turbulence which dissipates energy. The use of straight chamfers does not solve the problem associated with such turbulence.